Circulating platelets and vascular endothelial cells maintain a dynamic balance to promote platelet aggregation at a site of injury to the endothelium and to prevent thrombus formation in normal vessels. The interaction between these cells occurs in an environment where the local fluid mechanical conditions can play a major role in altering the responses of both cell types. Our hypothesis is that the interaction between the endothelial cells lining the vessel wall and circulating platelets depends on the local fluid shear stress and on the convective mass transfer of agonists between circulating cells and the vessel wall. We propose to investigate the interaction between platelets between platelets and endothelial cells in an in vitro parallel plate flow system that closely mimics the fluid mechanical environment found in the blood vessels. The level of platelet adhesion to intact and injured endothelial cell monolayers will be quantified using a video microscope and digital imaging system. The response of individual endothelial cells downstream from a mural thrombus forming at a site of damage to the endothelium will be determined by measuring endothelial cell cytosolic calcium levels, intracellular pH and membrane potential. Experiments will be performed using both steady and pulsatile flow. The experimental results will be compared with the numerical solution of the fluid flow and mass transfer equations. The results may allow us to identify which of the many substances released from a growing thrombus causes endothelial cells downstream to respond. This study will broaden our knowledge of the interaction between platelets and the vascular wall. Alterations in this interaction are an important contributor to vascular disease and bleeding disorders. By studying the interaction between platelets and endothelial cells in a system with well characterized fluid flow and mass transfer characteristics, we will be able to obtain a more complete understanding of the complex interactions which occur on the vascular wall in the presence of the flowing blood stream.